When a gas pipe breaks, several types of repairs may be desirable. If there is pressure in the pipe on both sides of the break, all that is necessary is to seal the pipe on both sides of the break. This allows service to be continued while the isolated section of pipe is repaired. Alternatively, gas may be fed through the pipe, under pressure, from a single source to consumers. In that event, if a break occurs between the source and the point of demand, and the pipe is sealed upstream of the break, all points downstream of the seal will be cut off from gas until the repair is made. If service is to be continued to consumers both upstream and downstream of the seal while the repair is being made, it is necessary to install a bypass around the break.
Gas pipes may be either metal or plastic, usually polyethylene. When a polyethylene gas pipe breaks, and the pipe is sealed so that it may be repaired, the seal is made by pinching. The apparatus necessary to effect this operation is quite large, since typical gas pipelines are about two inches in diameter and have a wall thickness of 3/16". Moreover, pinching the pipe in this manner and then reopening it by returning it by force to its initial round shape, weakens the pipe, making it vulnerable to breaks which may not occur until months later. This is particularly so in cold climates.
Machines exist whereby pipe may be sealed by boring a hole in the pipe on either side of the break, inserting a bypass stopper or plug through each of the holes, establishing a bypass connection between the two bypass plugs, repairing the break, and permanently sealing the inlets to the pipe where the bypass connection was made. Such machines include a fixture that permits a succession of tools to be applied to the pipe through a branching saddle while there is pressure in the pipe, the fixture including valving to permit the tools to be inserted and removed, notwithstanding the escape of gas under pressure from the pipe into the fixture. Existing fixtures of this type fail to maintain proper alignment between the pipe and the tools that are inserted into the pipe through the fixture. As a result, the tools tend to enter the pipe at an angle other than 90.degree. to the pipe's centerline, an undesirable condition. Moreover, such fixtures tend to bend the pipe while it is being worked on, because the fixtures are affixed to the pipe by means of a pair of brackets which straddle that portion of the fixture through which tools are inserted into the pipe. Consequently, as pressure is applied to the tools and through them to the pipe, the pipe bends, due to the bending moment between the tools and the brackets on either side of them.